It has been caused by chemical pollution of some kind, but the exact source of the contamination is yet to be determined.

Heathmont resident Julie West said the creek had been polluted before but “it’s never been this bad”.

“It’s environmental vandalism,” she said. “It’s like someone poured too much detergent into the water. The current is stirring up the suds and they’re in the trees, in the paddocks, they’re everywhere.”

She said onlookers had been saying that the area even smelled of detergent.

Friends of Dandenong Creek vice president Anthony Bigelow said it was the tenth time this year the waterway had been contaminated. “This weekend alone we have had three pollution events,” he said.

He said the creek had been foaming since Saturday, but was looking worse on Sunday, as these photos posted to social media show.

The creek on Saturday:

The foam on the creek on Saturday, May 19.Credit:First Friends of Dandenong Creek

And on Sunday:

Dandenong Creek on Sunday.Credit:First Friends of Dandenong Creek

But Mr Bigelow said the foam was “masking what’s happening down below” and a sign of high concentrations of chemicals being present in the water.

He said a five-kilometre stretch of the creek, from the Marie Wallace Park in Bayswater to Boronia Road in Vermont, was coated in white suds.

Mr Bigelow said he believed businesses based in the Bayswater industrial estate, where a stormwater drain feeds into the creek, were responsible for the pollution.

“What we fear is going to happen - like most of these events - is that the EPA will come out and say ‘source unknown’ and then close the case,” he said.

Mr Bigelow said animal life in the creek was unlikely to have been affected by the spill. But he said that’s only because the fish and other aquatic life in the creek were killed off by similar pollution in the area last November, and stocks had not since been replenished.

“We’re almost at the point of tears about this at the moment,” he said.

The Environment Protection Authority's executive director of regional services Damian Wells said the source and composition of the chemical pollution in the creek had not yet been determined.

Mr Wells said the authority had collected samples of the affected water for testing, which will help them understand the chemical make-up of the pollution and may help lead them to the culprit.

He said the spill appeared to contain detergents, which had become mixed in with the water and were also forming a film on the surface of the creek. He urged people in the Dandenong Creek area to avoid coming into contact with the polluted water or the foam.

Isolating the culprit of the pollution would be difficult, he said, because there were a lot of potential sources and tracking the path of the chemicals back to the origin would be a complex task.

He warned any rogue operators found responsible for the spill could face anything from a $8000 fine to prosecution in the courts, and urged anyone with information on the source of the pollution to contact the environmental authority's hotline on 1300 EPA VIC.

A Melbourne Water spokesman said crews were on-site to assist with the clean-up operation.